Apparatus and method for monitoring skin conductance and method for controlling a warning signal

ABSTRACT

The invention relates to an apparatus and a method for monitoring the autonomous nervous system in an individual, especially for detecting pain, by utilizing spontaneous change in skin conductance. The apparatus comprises measuring equipment ( 3, 4 ) for measuring the skin&#39;s conductance, and storage and processing means ( 5 ) for deriving secondary characteristics of the conductance signal, thus enabling it by means of signal means ( 6 ) to indicate that the pain or other activity in the autonomous nervous system has reached a certain threshold. The invention also relates to a method for controlling a warning signal ( 7 ) in such an apparatus.

The invention relates to an apparatus and a method for monitoring theautonomous nervous system of an individual, especially for detectingpain.

The invention also relates to a method for controlling a warning signalin an apparatus for monitoring the autonomous nervous system of anindividual, especially for detecting pain.

In the field of medical technology there is a problem in producingphysical measurements representing the activity in an individual'sautonomous nervous system, i.e. in the part of the nervous system whichis beyond the control of the will. It is particularly important toobtain indications of the activity in the sympathetic nervous system.For example, in various situations there is a need to monitor apatient's experience of pain.

There is a special need to monitor the sympathetic nervous system inbabies. It is known that babies with apnea (a cessation of breathing formore than 20 seconds) and attacks of lifelessness exhibit changes in thedegree of activity in the sympathetic nervous system. The indicationsare, therefore, that monitoring the sympathetic nervous system of babiesmay contribute towards revealing dysfunctions in the nervous system, andthat such monitoring can also be used to warn of the risk of cot death.

A method and an apparatus for monitoring the autonomous nervous systemin an individual will also be applicable in other situations. In thecase of both premature babies, infants and other individuals, bothchildren and adults, there may be a need to observe pain reactions. Thisapplies in particular to cases where the individual himself is unable toexpress the experience of pain in the normal manner, for exampleverbally, by means of crying or facial expression.

An example of this kind of application is monitoring of prematurebabies. These babies have a special need for pain monitoring, since theyhave poorly developed facial expression, in addition to which they maylack the energy to cry. In premature babies pain or stress have beenassociated with the occurrence of cerebral haemorrhage, and it has alsobeen shown that their experience of pain can be remembered, therebyaffecting their future pain reactions. It is therefore vital to know inreal time when such babies are exposed to pain, in order, amongst otherthings, to be able to administer analgesics.

Another application is monitoring of patients in a respirator.

The sympathetic nervous system can be activated by the feeling of pain,but also by other factors such as stress, fear and anger. When thesympathetic nervous system is activated, it causes reactions such asincreased heart rate, increased blood pressure and increased emotionalperspiration. Blood pressure, pulse and respiration rate are controlledboth by the sympathetic and parasympathetic nervous system, as well asbeing affected by other factors, such as if the patient loses a lot ofblood, or has lung or heart disease. Of all these phenomena, therefore,increased emotional perspiration is the most specific target for theactivity, particularly for detecting pain response in the sympatheticnervous system.

A known phenomenon associated with emotional perspiration is that theskin's conductance, particularly on specific parts of the body such asin the palms of the hands and the soles of the feet, is influenced bythe activity in the sympathetic nervous system, caused among otherthings by stimulation of the sense of pain. On exposure to pain stimulior other stress the sympathetic activity in efferent nerve fibres to thesweat glands increases. When the sympathetic nerves are activated, thesweat glands are activated, the sweat channels are filled with liquid,and the conductance in the skin increases. When the liquid evaporates,the skin conductance decreases. In this manner fluctuations arise in theskin conductance. This phenomenon is called spontaneous skinconductance. The spontaneous skin conductance consists of waves and abasal level. The number of waves and the height of the waves indicatedirect sympathetic activity in the nerves.

The basal level, however, does not constitute a satisfactory basis fordrawing conclusions concerning the activity of the patients sympatheticnervous system, including the presumed pain condition. Amongst otherthings this is due to the fact that after a pain or other nervous systemstimulation, the total skin moisture level and thereby the conductancesignal takes a relatively long time to return to its basal level (theso-called recovery time).

However, tests have shown that the skin's conductance appears as a timevariable signal which, in addition to a basal, slowly varying value (theso-called basal level), also has a component consisting of spontaneouswaves or fluctuations, in which characteristics of these fluctuations,such as for example their frequency and amplitude, are factors which arecorrelated with the experience of pain in the target object (thepatient). Measuring and analysing characteristics of these fluctuationsmay therefore be a suitable method of providing fast and reliableinformation concerning the activity in the sympathetic nervous system,including the effect of pain.

Tests which compare the activity in the sympathetic nervous system withskin conductance, where the frequency of maximum values and thefluctuations in the skin's conductance are taken into account, are knownfrom, amongst others, Edelberg R: Electrical Properties of the Skin.Methods in psychophysiology (Brown, C. C. (ed.)), Ch. 1, The Williams &Wilkins Company 1967, pp. 1-50, and from Lidberg L., Wallin G.Sympathetic skin nerve discharges in relation to amplitude of skinresistance responses. Psychophysiology 1981;18(3):268-270.

Previously known systems for analysing skin conductance comprise a dataacquisition system for recording a series of measurements for the skin'sconductance, and a computer for subsequent analysis of the recordedseries of measurements. The analysis therefore takes place after thedata have been collected, and thereby after the autonomous nervousactivity in the patient, as well as the sensations which gave rise tothe nervous activity, have already ceased. Such systems therefore offerno possibility of real time monitoring of the patient's autonomousnervous activity and pain reaction. In particular, they offer nopossibility of detecting and warning that a limit for pain reaction hasbeen exceeded during the period in which the measurements are performed.

From the patent literature several solutions are known which have somepoints of resemblance with the present invention:

U.S. Pat. No. 5,897,505 discloses a diagnostic apparatus for assessingpain in a human being, based on skin conductance and temperaturemeasurement. The apparatus is intended for performing a singlemeasurement at a time, and to indicate the result of this measurement ona display. The apparatus also includes auto-scaling and overrangefunctions which implies that two subsequent measurements are performed,in order to determine the appropriate scaling of the input signal andpossibly to indicate an overrange warning signal. The apparatus does notpermit the continuous monitoring and analysis of spontaneous response inskin conductance, particularly not the amplitude and frequency offluctuations in the skin conductance signal, caused by pain or similaractivities in the autonomous nervous system of the individual.

WO 85/00785 discloses an attention monitor based on skin resistancemeasurement. A warning signal is activated when the measured resistanceexceeds a predetermined amount. The publication does not indicate orsuggest the analysis of spontaneous response in skin conductance, causedby pain or similar activities in the autonomous nervous system of theindividual.

U.S. Pat. No. 4,697,599 discloses an example of a previously knownapparatus for detection of pain, based on measurement of the skin'sconductance. The apparatus exhibits a measurement on a display, inaddition to which it emits an audible signal which has a pulse frequencywhich varies in accordance with the measured conductance. The apparatusonly supplies information on the immediate value of the conductance, anddoes not analyse the measurements with regard to the frequency andamplitude of spontaneous fluctuations. The apparatus moreover uses adirect current-based resistance measurement, which can lead to sideeffects from the skin's polarisation properties.

GB-A-2.291.971 describes an example of an apparatus for visualbiofeedback therapy. The apparatus is based on the visualisation of anautonomous activity for the patient. This is achieved by a result of theautonomous activity being measured by a measuring device, which in thiscase may comprise a sensor for measuring the skin's conductance. Theresult of the measurement then influences an image which is displayed tothe patient. A feedback loop is thereby created which can enable thepatient to train himself to reproduce relaxation exercises which alterthe measured value in the desired direction. There is no indication thatthe apparatus comprises equipment for analysing spontaneousfluctuations, including their amplitude and frequency, in the measuredconductance signal. Nor is there any indication that the apparatus hasan area of application within the field of real time monitoring of apatient's sympathetic nervous system, including pain monitoring, but itis intended to be applied in the treatment of complaints such as, e.g.,irritable stomach/bowel syndrome.

WO 86/01317 describes a method and an apparatus for utilisingelectrodermal response as a control body for a computer. In this casethe skin's resistance is measured by means of a so-called paddle inputbody, equipped with electrodes, for a computer. In the method and theapparatus a certain amount of consideration is given to the dynamic ofthe resistance signal, not only the signal's immediate value, but alsothe signal change from one point of time to the next, being used in thecalculation of a control signal. However, the publication gives noanalysis of the amplitude and frequency of spontaneous signalfluctuations for monitoring purposes, and for detection of pain inparticular. In this publication too resistance measurement based ondirect current is employed, which may result in the skin's polarisationproperties influencing the measurements.

Related solutions can also be found in the part of the technology whichdeals with lie detectors. Some lie detectors employ skin conductance asthe basis of their analysis. However, such equipment does not make useof secondary characteristics in the skin conductance signal such as thefrequency and amplitude of spontaneous fluctuations for real timemonitoring of the activity in the sympathetic nervous system, and fordetection of pain in particular.

Thus an object of the present invention is to provide an apparatus formonitoring the autonomous nervous system in an individual, especiallyfor detecting pain, which is not encumbered by the disadvantages whichare mentioned above.

According to the invention this is achieved by an apparatus as mentionedabove, characterized in that it comprises the features which areindicated by the characterising part of the following independent claim1.

It is a further object of the present invention to provide a method forcontrolling a warning signal in an apparatus for monitoring theautonomous nervous system in an individual, especially for detectingpain, and which is not encumbered by the said disadvantages.

According to the invention this is achieved by a method as mentionedabove, characterized in that it comprises the features which areindicated by the characterising part of the following independent claim6.

Finally, it is an object of the present invention to provide a methodfor monitoring the autonomous nervous system in an individual,especially for detecting pain, and which is not encumbered by the saiddisadvantages.

According to the invention this is achieved by a method as mentionedabove, characterized in that it comprises the features which areindicated by the characterising part of the following independent claim10.

Further advantages and characteristics are indicated in the dependentclaims.

The apparatus and methods according to the invention will now bedescribed in more detail with reference to the attached drawings, inwhich

FIG. 1 illustrates a block diagram for an apparatus according to theinvention, and

FIG. 2 illustrates a flow chart for a method according to the invention.

FIG. 1 illustrates a block diagram for a preferred embodiment of anapparatus according to the invention. The apparatus is particularlyarranged for real time detection of pain reaction in an individual, forexample a patient. On an area 2 of the skin on a body part 1 of anindividual, sensor means 3 are placed for measuring the skin'sconductance. The body part 1 is preferably a hand or a foot, and thearea 2 of the skin on the body part 1 is preferably the palmar side ofthe hand (in the palm of the hand) or the plantar side of the foot(under the sole of the foot). The sensor means 3 advantageously comprisecontact electrodes where at least two electrodes are placed on the skinarea 2. In a preferred embodiment the sensor means 3 consist of threeelectrodes: a measuring electrode, a counter electrode and a referencevoltage electrode which ensures a constant application of voltage overthe stratum corneum (the surface layer of the skin) under the measuringelectrode. The measuring electrode and the counter electrode arepreferably placed on the skin area 2. The reference voltage electrodemay also be placed on the skin area 2, but it is preferably placed in anearby location, suitable for the measuring arrangement concerned.

In a preferred embodiment an alternating current is used for measuringthe skin's conductance. The alternating current advantageously has afrequency in the range up to 1000 Hz, corresponding to the area wherethe skin's conductance is approximately linear. A frequency should beselected which ensures that the measuring signal is influenced to theleast possible extent by interference from, e.g., the mains frequency.In a preferred embodiment the frequency is 88 Hz.

In the case of alternating current the conductance is identical to thereal part of the complex admittance, and therefore not necessarilyidentical with the inverse value of the resistance. An advantage ofusing alternating current instead of direct current in conductancemeasurement is that by this means one avoids the invidious effect on themeasurements of the skin's electrical polarising properties.

The resulting current through the measuring electrode is conveyed to ameasurement converter 4. This comprises a current to voltage converterwhich in a preferred embodiment is a transresistance amplifier, but inits simplest form may be a resistance which converts the current fromthe measuring electrode to a voltage. The measurement converter furthercomprises a decomposition circuit, preferably in the form of asynchronous rectifier which decomposes the complex admittance in a realpart (the conductance) and an imaginary part (the susceptance). However,it is sufficient if the decomposition circuit only comprises means forderiving the conductance.

The measurement converter 4 may also comprise amplifier and filtercircuits. In the preferred embodiment the measurement converter containsa low-pass filter. The object of the filter circuits may be partly todamp high-frequency noise, and partly to serve as anti-aliasing filtersfor preventing higher frequency components from being received bysubsequent circuits for time discretization. By means of the choice ofcomponents and design details, moreover, the measurement converter isdesigned with a view to obtaining high sensitivity and a low noiselevel.

The control unit 5 comprises means for time discretization of the signalfrom the measurement converter. The time discretization takes place at asampling rate which may advantageously be in the order of 20 to 200samplings per second. The control unit further comprises ananalog-digital converter, which converts measurement data to digitalform. The choice of circuits for time discretization and analog-digitalconvertion represent technical decisions for a person skilled in theart.

The control unit may advantageously comprise additional analog andpossibly also digital inputs, in addition to the input from themeasurement converter. In this case the control unit can either beequipped with a plurality of analog-digital converters, or it can employvarious multiplexing techniques well-known to those skilled in the artin order to increase the number of inputs. These additional analoginputs may, for example, be arranged for additional electrodermalmeasurements, or for other physiological measurements which mayadvantageously be performed simultaneously or parallel with theelectrodermal measurement, such as temperature, pulse, ECG, respiratoryand frequency measurements or oxygen saturation measurements in theblood.

The control unit 5 also comprises processing means for processing thedigitised measurement data, storage means in the form of at least onestore for storing data and programs, and adaptation circuits for givingat least one warning signal 7 to a signal unit 6, preferably also asignal for displaying information on display means 8. The control unit 5may also advantageously comprise a communication port for digitalcommunication with an external unit, such as a computer. Suchcommunication is well-suited for loading or altering the program whichis kept stored in the storage means in the control unit, or for addingor altering other data which are kept stored in the storage means in thecontrol unit, such as threshold values, which will be discussed later.Such communication is also well-suited for read-out of data from thestorage means in the apparatus, thus enabling them to be transferred toan external computer for further, subsequent analysis or storage. Acommunication port in the control unit will be advantageously designedin accordance with requirements for equipment safety for patients, asdescribed in more detail below.

In a preferred embodiment the storage means comprise a read-only storagein the form of programmable ROM circuits, containing at least a programcode and permanent data, and a read and write storage in the form of RAMcircuits, for storage of measurement data and other provisional data.

The control unit 5 also comprises an oscillator which delivers a clocksignal for controlling the processing means. The processing means alsocontain timing means in order to provide an expression of the currenttime, for use in the analysis of the electrodermal measurements. Suchtiming means are well-known to those skilled in the art, and are oftenincluded in microcontrollers or processor systems which are suitable foruse with the invention.

The control unit 5 may be realised as a microprocessor-based unit withconnected input, output, memory and other peripheral circuits, or it maybe realised as a microcontroller unit where some or all of the connectedcircuits are integrated. The means for time discretization and/oranalog-digital conversion may also be integrated in such a unit. Thechoice of a suitable form of control unit involves evaluations which arestandard for a person skilled in the art.

An alternative solution is to realise the control unit as a digitalsignal processor (DSP).

The control unit 5 is arranged to read in time-discrete and quantizedmeasurements for the skin conductance from the measurement converter 4,preferably by means of a program code which is stored in the storagemeans and which is executed by the processing means. It is furtherarranged to enable measurements to be stored in the storage means,preferably in a read and write store. The control unit is furtherarranged to analyse the measurements in real time, i.e. simultaneouslyor parallel with the performance of the measurements. In this context,simultaneously or parallel should be understood to mean simultaneouslyor parallel for practical purposes, viewed in connection with the timeconstants which are in the nature of the measurements. This means thatinput, storage and analysis can be undertaken in separate timeintervals, but in this case these time intervals, and the time betweenthem, are so short that the individual actions appear to occurconcurrently.

The control unit is further arranged to identify the fluctuations in thetime-discrete, quantized measuring signal, preferably by means of aprogram code which is stored in the storage means and which is executedby the processing means, and to derive analysis data expressing thefrequency and the amplitude of the fluctuations in the measuring signal.

In the control unit's storage means threshold data are storedrepresenting limiting values for the analysis data which are derivedfrom the measurements, corresponding to the conditions which will resultin the analysis of the measurements activating the warning signal 7 andthereby a signal device 6. The threshold data therefore correspond to apresumed limit for pain or similar activity in the individual'ssympathetic nervous system.

In the simplest form the threshold data can be universal, since theyapply to every individual on whom the apparatus or the method is used.The threshold data preferably constitute values characterising anindividual group to which the patient belongs. This grouping may dependon individual factors such as age, sex and/or possible diagnosis.Alternatively, the threshold data may constitute quite specific values,uniquely linked to the individual.

The processing means are arranged to compare the threshold data with thederived analysis data expressing the frequency and amplitude of thefluctuations in the measuring signal. The result of this comparisonexpresses whether or not the warning signal 7 will activate the signaldevice 6.

The warning signal 7 is preferably a digital signal, which indicatesthat the real time analysis of the skin conductance measurement hasresulted in a threshold for presumed pain reaction being exceeded. Thewarning signal 7 activates a signal or warning device 6, which may beany suitable device for visual or audible warning. Alternatively, thewarning signal may be transferred to or activate an externalcommunication or monitoring system, for example an external system forremote monitoring of patients.

In a special application of the invention the warning signal 7 oranother signal derived from the processing means in the analysis of theskin conductance measurements may be used to control an automaticadministration of a medication to the individual, particularly ananalgesic medication or a sleep-inducing medication. The signal may beused, for example, to control a pump for intravenous supply of morphine.In this case the invention will form part of a feedback loop for controlof the activity in the individual's autonomous nervous system.

In a preferred embodiment the display means 8 consist of a screen forgraphic visualisation of the conductance signal, and a digital displayfor displaying the frequency and amplitude of the measured signalfluctuations. The display units are preferably of a type whose powerconsumption is low, such as an LCD screen and LCD display. The displaymeans may be separate or integrated in one and the same unit.

The apparatus further comprises a power supply unit 9 for supplyingoperating voltage and current to the various parts of the apparatus. Thepower supply may be a battery or a mains supply of a known type.

The apparatus may advantageously be adapted to suit the requirementsregarding hospital equipment which ensures patient safety. Such safetyrequirements are relatively easy to fulfil if the apparatus isbattery-operated. If, on the other hand, the apparatus is mainsoperated, special requirements must be met by the power supply unit, orrequirements are made regarding a galvanic partition between parts ofthe apparatus (for example, battery operated) which are safe for thepatient and parts of the apparatus which are unsafe for the patient. Ifthe apparatus has to be connected to external equipment which is mainsoperated and unsafe for the patient, the connection between theapparatus which is safe for the patient and the unsafe externalequipment requires to be galvanically separated. Galvanic separation ofthis kind can advantageously be achieved by means of an opticalpartition. Safety requirements for equipment close to the patient andsolutions for fulfilling such requirements in an apparatus like that inthe present invention are well-known to those skilled in the art.

FIG. 2 illustrates a flow chart for a method for controlling a warningsignal in an apparatus for monitoring the autonomous nervous system ofan individual, and especially for detecting pain.

The method starts at reference 11.

Firstly, initial conditions 12 for the method are established. A vitalstep here is to establish threshold data, as mentioned above. Thesethreshold data constitute limiting values for the analysis data whichare derived from the measurements, corresponding to the conditions whichwill result in the real time analysis of the measurements activating thewarning signal 7 and thereby a signal device 6. Thus the threshold datacorrespond to a presumed limit for pain or similar activity in theindividual's sympathetic nervous system.

A repeated monitoring process is then carried out, comprising thefollowing actions:

The conductance of at least one area of the individual's skin ismeasured 13. The measurement may be continuous or time-discrete. In thepreferred embodiment the actual measurements are made with electrodes 3and a measurement converter 4 continuously in time, and the measuringsignal is made time-discrete in the subsequent control unit 5.

The measurements or data associated with the measurements at discretepoints of time are stored 14 in the storage means in the control unit.

An analysis 15 is made of current and previously recorded measurements.Analysis data is preferably derived expressing the frequency andamplitude of the fluctuations in the measuring signal. In a preferredembodiment these data are provided by considering the measuring signal'slocal maximum values and minimum values in a time window consisting ofan interval containing recently elapsed points of time. The existence ofa minimum and maximum value is established if the change in the signalvalue is essentially zero over a limited, recently elapsed period in theinterval.

In the interval analysis data are formed for the amplitude bycalculating the mean value of the differences from a minimum value tothe following maximum value. Analysis data for the frequency arepreferably formed by counting the number of maximum values contained inthe interval.

Alternatively, the measuring signal or a section of the measuringsignal's history can be decomposed (for example by digital filtering)into a slowly varying part, which constitutes the so-called basal linefor the measurements, and a more rapidly varying part, which constitutesthe fluctuations. Figures are then derived for the frequency andamplitude of the fluctuation component of the measuring signal.

The analysis of the measuring signal may involve more comprehensivemethods, including methods which transform from the time plane to thefrequency plane, for example real time Fourier transformation, wheresuch analysis also derives analysis data expressing the frequency andamplitude of the signal fluctuations.

The figures which are derived by the analysis are compared 16 with thestored threshold values. In the event of a negative comparison, whichmeans that the limit for pain or other activity in the individual'sautonomous nervous system has not been reached, the monitoring cycle isrepeated. In the event of a positive comparison, which indicates that alimit for pain or other autonomous nervous system activity has beenreached, the warning signal 7 is activated 17.

In the analysis it may be necessary to establish special criteria forwhen a maximum value should be considered valid. In their simplest formsuch criteria may be based on the fact that the signal has to exceed anabsolute limit in order to be able to be considered a maximum value. Inaddition, it is an advantage to base the criteria on the fact that thesignal has formed a peak which has lasted a certain time, thuspreventing extremely brief noise pulses from being considered as maximumvalues. The criteria may also be based on the fact that the increase inthe signal value as a function of time must remain below a certain limitif the maximum value is to be considered valid. The object is therebyachieved that artefacts which can occur in error situations such as,e.g., electrodes working loose from the skin, are not considered asvalid maximum values.

The above-mentioned criteria may be linked to the individual patient orto a group to which the patient belongs. This grouping is preferablybased on age, but may also include individual factors such as sex and/orpossible diagnosis.

A set of specially recommended criteria for validity of a maximum valueis as follows:

Absolute minimum limit for measured conductance: for adults 0.02 μS, forchildren 0-02 μS, for premature babies 0.015 μS

Minimum duration of a top: for adults 0.0 s, for children 0.0 s

Maximum increase just prior to the maximum value: for adults 2 μS/s, forchildren 2 μS/s

If necessary or desirable corresponding criteria may also be establishedfor the validity of minimum values.

The above description with drawings present a specific embodiment of theinvention, with the addition of some alternatives. For a person skilledin the art, however, it will be obvious that other, alternativeembodiments exist which are within the scope of the present invention,as indicated in the following patent claims.

What is claimed is:
 1. An apparatus for monitoring the autonomous nervous system in an individual for determining pain, comprising: measuring equipment for continuous or time-discrete measurement of the conductance of at least one area of the individual's skin, storage means for storing data associated with measurements of the conductance at discrete points of time, storage means for storing threshold data, processing means for analysing current and previous measurements of the conductance, and for comparing analysis data produced by this analysis with said threshold data, and signal means for indicating or warning when the comparison between said analysis data and said threshold data fulfils predetermined conditions indicative of pain.
 2. An apparatus according to claim 1, wherein the measuring equipment comprises a sensor comprising at least two electrodes, wherein said at least two electrodes are arranged to be placed on the area of the individual's skin where the conductance is to be measured.
 3. An apparatus according to claim 2, wherein the said processing means are also arranged to control a device for administering medication to the individual.
 4. An apparatus according to claim 1, wherein the measuring equipment comprises a measurement converter which provides conductance measurements by allowing at least a part of the area of the individual's skin to conduct an alternating current, by measuring the skin area's complex admittance, and by deriving the conductance component from the measured admittance.
 5. An apparatus according to claim 4, wherein the said processing means are also arranged to control a device for administering medication to the individual.
 6. An apparatus according to claim 1, wherein the processing means comprises means for deriving secondary characteristics of the measured and stored conductance signal in a time interval, wherein the secondary characteristics comprise the frequency and amplitude of fluctuations in the signal.
 7. An apparatus according to claim 6, wherein the said processing means are also arranged to control a device for administering medication to the individual.
 8. An apparatus according to claim 1, further comprising a means for displaying characteristics of the measured conductance signal, which characteristics include waveform of the signal, frequency of fluctuations in the signal and/or amplitude of fluctuations in the signal.
 9. An apparatus according to claim 8, wherein the said processing means are also arranged to control a device for administering medication to the individual.
 10. An apparatus according to claim 1, wherein the said processing means are also arranged to control a device for administering medication to the individual.
 11. A method for controlling a warning signal in an apparatus for monitoring the autonomous nervous system of an individual for determining pain, the method comprising steps of: initially establishing and retaining stored threshold data, thereafter implementing a monitoring process which comprises continuously or time-discretely measuring the conductance of at least one area of an individual's ski, storing measurements of the conductance, deriving analysis data for current and previous measurements of the conductance, comparing said analysis data with said threshold data, activating the warning signal if the comparison between said analysis data and said threshold data fulfils predetermined conditions indicative of pain.
 12. A method according to claim 11, the measuring step further comprises: allowing at least a part of the area of the individual's skin to conduct an alternating current, measuring the complex admittance of this part of the skin; and deriving the conductance component from the measured admittance.
 13. A method according to claim 12, wherein the derivation of analysis data from current and previous measurements comprises recording local maximum and minimum values for the signal in a time interval, and letting the analysis data represent the mean value of the differences from a maximum value to the following maximum value over the time interval, and the number of maximum values contained in the time interval. controlling a device for administering medication to the individual on the basis of derived analysis data.
 14. A method according to claim 12, further comprising: controlling a device for administering medication to the individual on the basis of derived analysis data.
 15. A method according to claim 11, wherein the deriving step further comprises: determining secondary characteristics of the signal which is composed of the current and previous measurements, which characteristics include the frequency and amplitude of fluctuations in the signal.
 16. A method according to claim 15, wherein the derivation of analysis data from current and previous measurements comprises recording local maximum and minimum values for the signal in a time interval, and letting the analysis data represent the mean value of the differences from a minimum value to the following maximum value over the time interval, and the number of maximum values contained in the time interval.
 17. A new method according to claim 15, further comprising: controlling a device for administering medication to the individual on the basis of derived analysis data.
 18. A method according to claim 11, wherein the derivation of analysis data from current and previous measurements comprises recording local maximum and minimum values for the signal in a time interval, and letting the analysis data represent the mean value of the differences from a minimum value to the following maximum value over the time interval, and the number of maximum values contained in the time interval.
 19. A method according to claim 18, further comprising: controlling a device for administering medication to the individual on the basis of derived analysis data.
 20. A method according to claim 11, further comprising: controlling a device for administering medication to the individual on the basis of derived analysis data.
 21. A method for monitoring the autonomous nervous system of an individual for determining pain, the method comprising steps of: initially establishing and retaining stored threshold data, thereafter implementing a monitoring process which comprises continuously or time-discretely measuring the conductance of at least one area of an individual's skin, storing measurements of the conductance, deriving analysis data for current and previous measurements of the conductance, comparing said analysis data with said threshold data, establishing increased activity in the autonomous nervous system in the individual, especially increased pain reaction, if the comparison between said analysis data and said threshold data fulfils predetermined conditions indicative of pain. 